Contactless power transmission is recently being used as a method for charging secondary batteries in cellular phones, digital cameras, and the like. Electric field coupling, in which power is transmitted through capacitive coupling between opposing electrodes, is one technique used for contactless power transmission. In contactless power transmission that uses electric field coupling, high voltages and high frequencies are important for increasing the efficiency of power transmission. As such, in order to employ electric field coupling contactless power transmission for electronic devices such as cellular phones, it is necessary to provide, in an electronic device serving as a receiving side, a transformer that transforms high-voltage power supplied from a sending side through contactless power transmission into low-voltage power suited to the circuitry of the electronic device.
A piezoelectric transformer is an example of a transformer that can be used as a transformer for contactless power transmission. The piezoelectric transformers disclosed in Patent Document 1 and Patent Document 2 are known as examples of conventional piezoelectric transformers. However, the piezoelectric transformers disclosed in Patent Document 1 and Patent Document 2 are piezoelectric transformers for cold-cathode tubes used in liquid crystal display backlights. A cold-cathode tube piezoelectric transformer handles lower frequencies than the frequencies demanded from transformers used for contactless power transmission. Accordingly, the piezoelectric transformers disclosed in Patent Document 1 and Patent Document 2 cannot be used as-is as transformers for contactless power transmission. Specifically, there are two ways in which the piezoelectric transformers disclosed in Patent Document 1 and Patent Document 2 can be altered to handle the high frequencies demanded from transformers for contactless power transmission. One is by further reducing the size of the piezoelectric transformer, and the other is by changing the vibration mode to a higher-order vibration mode. With the former, further reducing the size of the piezoelectric transformers disclosed in Patent Document 1 and Patent Document 2 reduces the efficiency of power transmission. Accordingly, it is necessary to change the vibration mode to a higher-order vibration mode in order to handle the high frequencies demanded from transformers for contactless power transmission without a drop in the efficiency of power transmission. A coil transformer is another example, aside from a piezoelectric transformer, of a transformer that can be used as a transformer for contactless power transmission. While coil transformers are currently the mainstream in terms of transformers for contactless power transmission, they are also larger than piezoelectric transformers. Coil transformers furthermore carry a risk of increased electrical resistance as the frequency of supplied power rises.
For such reasons, a piezoelectric transformer that employs a high-order vibration mode is in demand as a transformer for contactless power transmission. A piezoelectric transformer that employs primary to tertiary vibration modes will be given here as an example in order to describe a problem with piezoelectric transformers that employ high-order vibration modes.
A piezoelectric transformer 500 illustrated in FIG. 12 is a piezoelectric transformer that uses a primary (base) vibration mode or a secondary vibration mode. A piezoelectric transformer 600 illustrated in FIG. 13 is a piezoelectric transformer that uses a tertiary vibration mode. FIG. 12 is a diagram illustrating a side surface of the piezoelectric transformer 500 along with stress and displacement in respective areas of the piezoelectric transformer 500. FIG. 13 is a diagram illustrating a side surface of the piezoelectric transformer 600 along with stress and displacement in respective areas of the piezoelectric transformer 600. The arrows in FIG. 12 and FIG. 13 indicate polarization directions. A graph in (a) of FIG. 12 represents stress W1a and displacement W1b in the respective areas in the primary vibration mode, and a graph in (b) of FIG. 12 represents a waveform W2a indicating stress and W2b indicating displacement in the respective areas during the secondary vibration mode. A graph in FIG. 13 represents stress W3a and displacement W3b in the respective areas during the tertiary vibration mode.
As shown in FIG. 12, the piezoelectric transformer 500 includes a long, plate-shaped piezoelectric body 501 configured of piezoelectric ceramics, an input electrode 520, and an output electrode 530. The input electrode 520 is provided on two main surfaces of one side of the piezoelectric body 501. The output electrode 530 is provided on an end surface of the other side of the piezoelectric body 501. As shown in FIG. 12, the one side of the piezoelectric body 501 is polarized along a thickness direction of the piezoelectric body 501, and the other side of the piezoelectric body 501 is polarized along a lengthwise direction of the piezoelectric body 501.
According to the piezoelectric transformer 500, when a voltage at a specific frequency is applied to the input electrode 520, a strong mechanical vibration is produced in the piezoelectric body 501 due to an inverse piezoelectric effect. A standing wave having a half-wave length is produced in the piezoelectric body 501 at this time. Furthermore, the piezoelectric transformer 500 outputs a voltage corresponding to the mechanical vibrations from the output electrode 530 as a result of a piezoelectric effect.
As shown in FIG. 13, the piezoelectric transformer 600 includes a long, plate-shaped piezoelectric body 601 configured of piezoelectric ceramics, input electrodes 620, and output electrodes 630. The input electrodes 620 are provided in a central area of two main surfaces of the piezoelectric body 601. The output electrodes 630 are provided on both end surfaces of the piezoelectric body 601 in the lengthwise direction thereof. The central area of the piezoelectric body 601 is polarized along a thickness direction of the piezoelectric body 601, and both end portions of the piezoelectric body 601 are polarized along the lengthwise direction of the piezoelectric body 601.
According to the piezoelectric transformer 600, when a voltage at a specific frequency is applied to the input electrode 620, a strong mechanical vibration is produced in the piezoelectric body 601 due to an inverse piezoelectric effect. A standing wave having a 1.5 wavelength is produced in the piezoelectric body 601 at this time. Furthermore, the piezoelectric transformer 600 outputs a voltage corresponding to the mechanical vibrations from the output electrode 630 as a result of a piezoelectric effect.
The respective waveforms shown in FIG. 12 and FIG. 13 will be compared next. Specifically, the waveforms W1b, W2b, and W3b in the respective vibration modes will be compared, by comparing the positions of the apexes of the antinodes where the phase of the displacement waveform reaches 180°, using a point at a left end portion where the displacement is equal as a base point (phase=0°). A distance between the position of the apex of the antinode in the secondary vibration mode and the position of the apex of the antinode in the tertiary vibration mode is shorter than a distance between the position of the apex of the antinode in the primary vibration mode and the position of the apex of the antinode in the secondary vibration mode. This indicates that the wavelength becomes shorter in higher-order vibration modes, and the apexes of the antinodes near the base point become closer in the displacement waveforms in the respective vibration modes. In other words, with a piezoelectric transformer that uses high-order vibration modes, a plurality of vibration modes having similar waveforms will be present together. As such, if an attempt is made to excite a desired vibration mode in a piezoelectric transformer that uses high-order vibration modes, it is possible that a different vibration mode having a waveform similar to the waveform of the desired vibration mode will be excited instead. In other words, piezoelectric transformers that use high-order vibration modes have a problem in that unnecessary vibration modes occur with greater ease than in piezoelectric transformers that use low-order vibration modes.
Patent Document 1: Japanese Patent No. 2998717
Patent Document 2: Japanese Patent No. 4297388